1. Field of the Invention
This invention relates generally to the prevention of bleeding of organic material onto a metal substrate and, more particularly, to preventing bleeding of adhesive or components thereof onto a noble metal substrate. In even more particular aspects, this invention relates to the prevention of the bleeding of any components of a die attach adhesive onto noble metal circuitry on a dielectric substrate and, specifically, onto gold wire bond surfaces on a chip carrier or other noble metal circuitry on an I/C chip mounting substrate.
2. Background Information
In the mounting of integrated circuit (I/C) chips on substrates having electrical connections, such as wire bond pads thereon, one technique of mounting utilizes a two-part epoxy adhesive to bond the chip either to gold plating on a planar surface of a substrate, or onto copper or some dielectric material in a cavity formed in an electrical substrate, which substrate also has electrical contacts or wire bond surfaces on the surface thereof. One particular problem encountered when using such bond techniques is the tendency of the components of the epoxy adhesive to xe2x80x9cbleedxe2x80x9d and spread onto the wire bond surfaces, such as the wire bond pads or other noble metal surfaces, such as ground rings or voltage rings, and adhere thereto. This xe2x80x9cbleedxe2x80x9d causes the surface of the bond pads or other metal surfaces to be unreceptive to the bonding of the wire bonds necessary to electrically connect the wire bond surfaces to the contacts on the I/C chip. This, of course, results in unsatisfactory electrical contacts and, thus, contributes to an unsatisfactory chip package.
One prior art technique for solving this problem is to treat the part with a CF4 plasma. The plasma reacts with the organic portion of the substrate, and the fluorine containing moieties settle on the gold surface of the wire bond pads and other gold surfaces and make them less susceptible to being wetted by bleed from the adhesive. However, this is not a completely satisfactory treatment in all instances in that the fluorine containing moieties are not strongly bonded to the gold but are only lightly adhered thereto. Therefore, they can be easily removed with organic solvents during processing before attachment of the I/C chip takes place, but not easily removed with aqueous systems during processing. Moreover, the CF4 plasma tends, in many cases, to adversely affect the dielectric material for subsequent application of encapsulating material.
It is, therefore, an object of the present invention to provide a method and structure for substantially reducing or eliminating the wettability of metal surfaces, particularly noble metal surfaces, and specifically gold or other noble metal surfaces, to organic materials, particularly any components of an adhesive, such as an epoxy used for attaching an I/C chip to a substrate, and which I/C chip is subsequently bonded thereto.
According to the present invention, a method and structure for preventing wetting or bleed of an adhesive, such as an epoxy, onto a metal surface and particularly wire bond surfaces on the surface of a dielectric substrate is provided. This invention is particularly useful in preventing bleed from any components of an adhesive for attaching an I/C chip or a die to a substrate, and more particularly when a two-part epoxy adhesive is used. The method includes treating the surfaces of the wire bond pads with a chemical composition that prevents wetting of, or bleed onto, the wire bond surfaces by a component of the epoxy used to attach the die to the substrate surface. The chemical composition is a chemical which will provide xe2x80x9cSelf-Assembled Monolayersxe2x80x9d (SAMs) on the surface of the gold. These compositions are characterized by a molecule having at least one group, such as a mercaptan or disulfide, connected to a hydrocarbon or substituted hydrocarbon moiety, such as (CH2)n chain. A particular molecule that is useful for this is 1-octadecanethiol [HSxe2x80x94(CH2)17xe2x80x94CH3]. The thiol or sulfur-containing portion of the molecule has an affinity for gold or other noble metals, such as palladium, platinum and silver, and chemically bonds to the metal surface, leaving the hydrocarbon segment oriented away from the surface. The affinity of the thiol or sulfur-containing portion of the molecule chemically bonding with the metals provides a relatively strong attachment of the molecule to the metal surface. The hydrocarbon segment oriented away from the surface presents a surface on the metal which has the characteristics of the hydrocarbon portion of the molecule which has a low surface tension, and, thus, prevents wetting of the gold or other noble metal by an epoxy adhesive component. Other chemical moieties, such as disulfides having a general formula of Rxe2x80x94Sxe2x80x94Sxe2x80x94Rxe2x80x2, where R and Rxe2x80x2 are the same or different hydrocarbon moieties, such as (CH2)x or substituted chains, could be used to form the SAMs.
The SAMs, once they provide protection from the bleed of the die attach adhesives, self desorb or are otherwise removed from the gold surface during the wire bonding temperatures, thus rendering the gold surface free from any organic layer which would interfere with wire bonding process. The composition is applied as a coating, preferably in a solvent such as butylcarbitol.